A phased array or smart antenna is an electronically steerable directional antenna used for example in radar or in wireless communication systems. A phased array receiver contains a group or matrix of antenna elements and associated receive channel circuits in which the relative phases of received signals are varied in such a way that the effective reception pattern of the array is directed in a desired direction and suppressed in undesired directions. Beamforming is used in sensor arrays for directional signal transmission or reception. This spatial selectivity is achieved by using adaptive or fixed receive/transmit beam patterns.
A signal may be a time varying physical quantity carrying information, e.g. a varying voltage level, for example occurring at an antenna element when receiving an electromagnetic wave.
A phased-array receiver employing beamforming controls the phase and relative amplitude of the incoming signal received by each antenna element and combines the output signals delivered by receive channels associated with each antenna element in such a way that a particular radiation pattern can preferentially be observed. A phased-array receiver uses the phase difference of an incoming signal received at different antenna elements of an antenna matrix to determine the angular position of a target. Phased-array receivers are for example used in radar systems, where the bandwidth is a fraction of the centre frequency, and a time delay introduced by the spatial distance between receiving antenna elements can be mapped to a phase-shift between received signals. The spatial difference translates to a phase difference of the output signals of the receive channels, which may for example be downconverted signals, i.e. may have a frequency below the frequency of the received signal. This principle can be used to steer the antenna beam by introducing a phase-shift at the receive channels. This phase-shift can be realized using digital beamforming (DBF) in the digital domain, i.e. after sampling, or using analog beamforming (ABF) in the analog domain, i.e. prior to sampling.
Conversion of the incoming signal into a different frequency range may be achieved by means of a mixing circuit arranged to mix the incoming or received signal with a signal generated by a local oscillator circuit.
As shown in FIG. 1, a first prior art receive channel 100 of a first phased-array receiver can contain an antenna element 110 which delivers a radio frequency (RF) signal to a phase-shifter circuit 112 arranged to phase-shift the signal in the RF domain and provide it to mixer circuit 114 coupled to a local oscillator (LO) circuit 116. A generated output signal is delivered for analog or digital post-processing 118. Post-processing may include summing output signals delivered by multiple receive channel. Alternatively, as shown in FIG. 2, a second prior art phased-array receiver 200 has multiple receive channels, each containing an antenna element 210, 220, 224, which deliver radio frequency signals (RF1, RF2, RF3) to corresponding phase-shifter circuits 212, 222, 226 arranged to phase-shift the signals and provide them to a summation circuit 228 for power combining in the RF domain before provision to a mixer circuit 214 coupled to a local oscillator circuit 216. A generated output signal is then delivered for analog or digital post-processing 218.
In Koh et. al. “An X- and Ku-Band 8-Element Phased-Array Receiver in 0.18-μm SiGe BiCMOS Technology”, IEEE Journal of Solid-State Circuits, Vol. 43, No. 6, June 2008, pp. 1360-1371, a phased-array receiver implemented using an all-RF architecture is shown, where the phase-shifting and power combining is carried out at the RF-level. Similarly, in Koh et. al. “A Q-Band Four-Element Phased-Array Front-End Receiver with Integrated Wilkinson Power Combiners in 0.18-μm SiGe BiCMOS Technology”, IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 9, September 2008, pp. 2046-2053, an all-RF architecture is shown, where phase-shifting is applied in the received signal path.
In Yu et al. “A 22-24 GHz 4-Element CMOS Phased Array With On-Chip Coupling Characterization”IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008, pp. 2134-2143, the incoming signal is split into an in-phase (I) and a phase-shifted quadrature (Q) component. Generation of I and Q signals and summing is done in the RF signal path.
As shown in FIG. 3, a third prior art receive channel 300 of a third phased-array receiver can contain an antenna element 310 which delivers a radio frequency (RF) signal to mixer circuit 314. A generated output signal is delivered for analog or digital post-processing 318. Local oscillator circuit 316 is coupled to mixer circuit 314 through a phase-shifter circuit 312 arranged to phase-shift the local oscillator (LO) signal, i.e. phase-shifting may alternatively be performed not in the received signal path, but in the LO path before applying the LO signal to a mixer device for mixing with the received RF signal. In Jeon et al., “A Scalable 6-to-18 GHz Concurrent Dual-Band Quad-Beam Phased-Array Receiver in CMOS”, IEEE Journal of Solid-State Circuits, Vol. 43, No. 12, December 2008, pp. 2660-2673, an integrated phased-array receiver is shown, wherein phase-shifting is performed in the local oscillator path.
Referring to FIG. 4, a prior art receive channel 400 is shown, wherein an antenna element 410 delivers a radio frequency (RF) signal to a power splitter circuit 412. In-phase (I) and quadrature (Q) signals are generated by mixing with a local oscillator 416 signal using directional coupler 418 and I- and Q-mixers 414, 424. I- and Q-signals are separately applied to dedicated analog-to-digital converters (ADC) 420, 422 for sampling the I- and Q-signals and subsequent digital processing. In PCT/US2006/046792, a linear FM radar system is presented, wherein two analog-to-digital conversion (ADC) circuits are used for sampling the output signals of an IQ-mixer.
As shown in FIG. 5, another prior art receive channel 500 of a phased-array receiver may contain a mixer circuit 514 for mixing received radio frequency (RF) signals received at antenna element 510 with local oscillator 516 signal in order to frequency-shift the received signal to a different frequency, for example an intermediate frequency (IF) below RF. The generated IF signal may then be split using an IQ generation module 518 into an in-phase signal 520 and a 90° phase-shifted quadrature signal 522 which may then be applied to a vector modulator or phase-shifter or phase rotator 512 and a weighting amplifier 524. In other words, the incoming RF signal is first downconverted into an IF signal, and IQ-generation and phase-shifting is applied afterwards in the IF domain.
In PCT/GB95/01607, a circuit module for a phased-array radar is shown, wherein superheterodyne, single-sideband receive channel modules with dedicated ADCs for each I- and Q-output are used. In Erkens et al., “A Low-Cost, High Resolution, 360° Phase/Gain Shifter in SiGe BiCMOS”, IEEE 2009, polyphase filters are used for I- and Q-signal generation.